On Sun, 9 Jul 2006 20:57:18 +0100, "David" nospam@nospam wrote:

Can someone provide a full description of how a quarterwave vertical antenna
with radials works? Length of radials is also a quarterwave.

Hi David,

Someone can, and someone already has, but that hasn't helped you has
it? The following statements suggest so:

I find that many books give a good description of antennas like the Yagi,
and then suddenly become very vague when describing the quarterwave
vertical.

It isn't vague, unless you've been saddled with poor references. On
the other hand there is not much to say when you are working with
elementary monopoles and dipoles. Yagis, in this sense, have much to
be discussed.

Books
refer to image theory where an image of the radiating element is produced by
the radials, and show a spear shape going into the ground. Some say the
radials are the other half of a dipole.

Radials being the "other half" simply reveals that the monopole
(especially when elevated) is a vertical dipole.


What difference does it make if the radials are in free space or in the
ground?

About 3dB.

Some articles claim that the radials tend not radiate because they
cancel out,

All parts of an antenna radiates, the radials' contributions cancel -
at a distance.

while other other articles claim that the radials simulate a
ground plane and reflect the radio wave. Can you explain this contradiction?

Poor references. The radials simply serve for drivepoint Z
consideration (we already agree that their contribution to radiation
cancel). For all practical purposes, the "ground plane" would have to
extend out 5 to more wavelengths to affect the lobe characteristics of
radiation.

The vertical element is usually called the radiating element. How well do
the radials radiate?

Perfectly, or as well as the "radiator" presuming they all exhibit
similar construction.

The same magnitude of current flows into the vertical
element as the radials, although the current into the radials is split.

A normal ground plane is a large sheet of metal that reflects the radio wave
emitted by the radiating element. If there are four radials, each a
quarterwave long, do the radials form a ground plane? Or is there too much
of a gap for them to form a ground plane?

They are simply not long enough, and certainly don't exhibit near the
coverage (the gap you describe) as does a plane of metal (or
seawater).

If the radials are disconnected and taken away, with the vertical
quarterwave element still connected to centre conductor, do I still have a
radiating element?

A poor one, but given the wheel of fortune, the feedline could make up
the difference.

What happens to the SWR?

It usually goes ballistic, but again, with ground nearby, you could be
heating worms and find the SWR at a comfortable value.

73's
Richard Clark, KB7QHC